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No context or other column names in the question except PKs. I'm assuming tag_map is a many<>many relationship between forum/blog posts and tags that have been associated with each post?
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PhilMay 13 '12 at 17:25

"and order them by PRIMARY KEY" No it won't! There's no ORDER BY so the results won't be ordered.
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JannesMay 14 '12 at 8:55

Also, with your query a post with 3 tags will be in the resultset 3 times! So you probably want to add a GROUP BY post_id anyway. Then just a COUNT(*) AS cnt in the select and add ORDER BY cnt DESC.
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JannesMay 14 '12 at 9:03

@Jannes it will not ORDER by PRIMARY KEY, but the results will be retrieved in this order (i.e. order of INSERT). At least in my experience (maybe by chance ;)
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AllMay 14 '12 at 11:41

I've made a test with the index. At this data quantity there's obviously no improvement in execution time, but I think the Execution Plan shows an improvement (on the last line). I've added a link to the new SQLFiddle in my post
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w0lfMay 17 '12 at 19:11